1. Field of the Invention
The present invention relates to a gas sensor element and a gas sensor using the same, more particularly, to a gas sensor element having at least two-cell configuration suitable for use as, for example, a full-range air-fuel ratio sensor (a sensor capable of measuring air-fuel ratio of an internal combustion over the entire range thereof), a nitrogen oxide gas sensor, a flammable gas sensor (a sensor capable of measuring carbon monoxide, hydrocarbon gas, or other flammable gas), or a compound gas sensor (a sensor capable of measuring a plurality of gases selected from among oxygen, nitrogen oxides, carbon monoxide, hydrocarbon gas, and other flammable gases).
2. Description of the Related Art
Increasingly strict regulations have been imposed on the emission quantity of harmful matter (e.g., hydrocarbon gas, carbon monoxide, and nitrogen oxides) contained in exhaust gas discharged from an internal combustion engine of an automobile, etc. Moreover, in view of the greenhouse effect and other problems, the necessity to reduce the emission of carbon dioxide has arisen, thereby raising an urgent need for a method of further reducing consumption of fuel by internal combustion engines.
Under such circumstances, more strict requirements have been imposed on gas sensors, which are indispensable to reduction of harmful matter contained in exhaust gas and improvement of fuel efficiency, and gas sensors having improved performance and reliability are demanded. In particular, in recent years, demand has arisen for a gas sensor which can be activated quickly and can save electric power, while having improved performance and reliability. Moreover, carbon monoxide gas sensors and nitrogen oxide gas sensors capable of directly detecting harmful components themselves have attracted a great deal of attention.
The below-mentioned patent documents 1 and 2 and other documents disclose a two-cell-type gas sensor element which includes an oxygen pump cell having a pair of electrodes provided on a solid electrolyte layer, and an oxygen detection cell having a similar configuration. The gas sensor element enables full-range measurement of air-fuel ratio of an internal combustion engine of, for example, an automobile, to thereby improve fuel efficiency of the internal combustion engine. In a gas sensor element of such a type, ions such as oxygen ions must quickly move through the solid electrolyte layer. Therefore, a heating resistor for heating the solid electrolyte layer is disposed in the vicinity of the cells in order to heat and maintain the oxygen pump cell at a temperature of, for example, 700° C. or higher. However, since ceramic material which constitutes the solid electrolyte generally has low thermal conductivity, the operation of heating the cells to high temperature for, in particular, quick activation of the cells cannot be performed quickly and efficiently. Moreover, since use of a large amount of expensive zirconia material increases the cost of such a gas sensor, the range of application thereof is limited, even though such a gas sensor exhibits excellent performance.
Notably, the below-mentioned patent documents 3 and 4 disclose a technique for simultaneously firing zirconia ceramic and alumina ceramic. Moreover, the below-mentioned patent document 5 discloses a technique for preventing migration.
[Patent Document 1]
Japanese Patent Application Laid-Open No. 62-148849
[Patent Document 2]
Japanese Patent Application Laid-Open No. 11-14594
[Patent Document 3]
Japanese Patent Application Laid-Open No. 2001-66280
[Patent Document 4]
Japanese Patent Application Laid-Open No. 2000-292406
[Patent Document 5]
Japanese Patent Application Laid-Open No. 62-44971
3. Problems Solved by the Invention
Among the above-described problems, the problem in relation to heating efficiency can be solved by a gas sensor element disclosed in patent document 2. The gas sensor element utilizes a substrate in which a heating resistor is embedded in a ceramic material that contains alumina, having excellent thermal conductivity, as a predominant component. However, this gas sensor has the following drawback. Since the substrate is formed of alumina which greatly differs in thermal expansion from zirconia that is used as a material for a solid electrolyte layer, when the substrate and the solid electrolyte layer are joined through simultaneous firing, cracks are generated in the solid electrolyte layer or the solid electrolyte layer is broken into pieces. Further, patent documents 4 and 5 disclose a gas sensor that is fabricated through simultaneous firing (namely cofiring) of an alumina substrate and a solid electrolyte layer containing alumina and zirconia. However, few investigators have reported a sensor structure in which both an oxygen concentration detection cell and an oxygen pump cell are formed on the same surface of a solid electrolyte layer, wherein one of electrodes of each cell is disposed within a common gas diffusion chamber and the other electrode is formed outside the gas diffusion chamber, and wherein the pump cell which utilizes oxygen ions flowing in a direction of the planer surface of the solid electrolyte layer is disposed on a common plane of the alumina substrate). Moreover, few investigators have reported technical problems arising from such a two-cell sensor structure having at least a pair of pump cell electrodes formed on a planer electrolyte layer surface.